The present invention relates generally to digital communication networks, and more specifically, to an efficient framing procedure for variable length packets.
Efficient transfer of traffic requires a network designed in conformance with conventional voice network and suitable for transferring variable length packets. Conventionally, there is SONET/SDH as a digital network for WAN (Wide Area Network). SONET and SDH are a set of related standards for synchronous data transmission over fiber optic networks. SONET is short for Synchronous Optical Network and SDH is an acronym for Synchronous Digital Hierarchy. SONET is the United States version of the standard published by the American National Standards Institute (ANSI). SDH is the international version of the standard published by the International Telecommunications Union (ITU).
Increase in bandwidth demand has been satisfied through a combination of increased line rates (Time Division Multiplexing (TDM)) and transmitting multiple wavelengths through a single fiber (Dense Wave Division Multiplexing (DWDM)). ITU recommendation G.709 (“Interface for the Optical Transport Network (OTN))” builds on the experience and benefits gained from SDH and SONET to provide a route to the next-generation optical network. The ITU-T G.709 frame includes three parts: overhead area for operation, administration, and maintenance functions; payload area for customer data; and forward error correction (FEC). FEC provides additional coded data to enable error checking and correction by a receiving device. There are also conventional FEC implementations based on ITU-T G.975 which support only the transmission of SONET/SDH signal and can not carry OPU.
ITU-T G.709 links running at an appropriate rate can carry 2.5 Gb/s Ethernet (2.5 Gb/s Ethernet is not included in the IEEE 802.3 standard but may be used in proprietary systems, for example), 10 Gb/s Ethernet, and future rate Ethernet. However, there are difficulties with the transparent transport of 10 GE (Gigabit Ethernet) interface, which may or may not transport CDL information (described below), over a WDM system that uses FEC to improve optical system performance. Currently there are two different approaches to solve this problem. One is based on the over clocking of the G.709 frame rate that runs at 10.709 Gb/s at a rate up to 11.09 Gb/s in order to transport the 10.3 Gb/s client signal. The second is the use of ITU-T G.7041 mapping over ITU-T G.709. Both of them have drawbacks. For example, ITU-T G.709 overclocking requires a higher bit rate, which may result in system and hardware problems. ITU-T G.7041 GFP mapping does not provide preamble transparency which creates compatibility issues for technologies such as converged data link (CDL) Ethernet (described in U.S. patent application Ser. No. 09/668,253, filed Sep. 21, 2000, which is incorporated by reference herein in its entirety). IEEE is in the process of standardizing uses of the Ethernet preamble for technologies such as CDL Ethernet. Transport of CDL Ethernet over WDM or ultra-long reach links results in additional requirements. For example, starting at 10 Gb/s for ultra-long reach transmission and for future higher bit rates for even longer reach transmission, FEC is required. Furthermore, as optical switching without OEO conversion is deployed, some OAM information pertaining to all the wavelengths needs to be carried on a separate supervisory wavelength.
Unlike SONET, Ethernet networks rely on non-synchronous signaling techniques. Gigabit Ethernet uses the same frame format specified by the original Ethernet Standard, including variable frame length specified in the Ethernet Standard. Framing procedures have been developed to accommodate variable-length packets with various protocols in an OTN using WDM in addition to SONET/SDH. However, existing framing procedures (e.g., HDLC, Generic Framing Procedure Transparent Mode, Generic Framing Procedure Fame Mode) suffer from disadvantages such as variable bandwidth overhead and high overhead that may result in bandwidth limitation and transparency issues. The high overhead makes it virtually impossible to carry a full 10 Gb/s Ethernet signal in SONET OC-192C or OTN OTU2 byte containers. For example, OTU2 cannot carry 10 Gb/s Ethernet using GFP transparent mode. OPU2 payload capacity is not sufficient for direct mapping of 64B/66B code words. GFP Frame Mapped Mode cannot be used to transport CDL Ethernet, since the Ethernet preamble is discarded. Furthermore, extensions to the GFP Frame Mapped Mode maintain its high overhead. These overheads are provided to support capabilities that are redundant to CDL Ethernet. For example, GFP Frame Mapped Mode defines an idle packet. CDL Ethernet already includes an idle packet. Furthermore, GFP Frame Mapped Mode defines client management packets which compete with client packets for bandwidth or have an indeterminate latency. In the case of FEC based on G.975 that support only OC-192/5TM-16 signal, the GFP frame mapping over SONET/SDH payload will reduce the available bandwidth down to 9.584 Gb/s.
Further drawbacks to GFP include the rate control (e.g. the open-loop rate control in 10 Gb/s WAN PHY and compatible MAC) which is required to carry 10 Gb/s Ethernet into OC192. GFP also requires use of the linear extension header and therefore even larger overhead and lowering of the rate to support packet-by-packet multiplexing. Moreover, transport network elements are needed by GFP to inject frames into the client frame stream for OAM&P. In the case of FEC based on G.975 that support only OC-192/STM-16 signal, the GFP frame mapping over SONET/SDH payload will reduce the available bandwidth down to 9.584 Gb/s.
In the absence of a suitable encapsulation/mapping procedure, and to meet the above objectives, an efficient framing procedure for Ethernet is needed.